1. Field of the Invention
The invention relates generally to analyzers for charged particle beams. In particular, the invention relates to energy analyzers of electrons or other charged particles.
2. Background Art
The parent application Ser. No. 10/618,078, incorporated herein by reference in its entirety, describes a spectrometer in which the energy of secondary electrons is measured in the range of a few electron volts (eV) to a few keV. There are many types of such spectrometers used for characterizing the composition and other properties of materials in which the electron energy needs to be measured, for example, X-ray photoelectron spectrometers and electron spectrometers, and secondary ion spectrometers. Similar spectroscopes, such as secondary ion spectrometers, have been applied to other charged particles, such as energetic ions. Many scientific experiments require accurate measurement of the energy distribution of charged particles. The parent application was principally directed to a vacuum window usable on electron spectrometers that is transmissive to such low-energy electrons. In contrast, the present application is principally directed to the charged particle electrostatic optics used in a charged particle energy analyzer to discriminate the energy of charged particles and to measure their intensity or flux.
The performance of a charged particle energy analyzer, of which an electron energy analyzer is but one example but the most prevalent example, is gauged by several conflicting characteristics. It needs to have a narrow resolution over a reasonably large energy band and the selected energy should be easily tuned. Its resolution needs to be stable and not require repeated calibration. The energy analyzer needs to have a high detection efficiency, which results in a high throughput of analyzed samples. Of especial importance in material characterization in which secondary electrons or ions are emitted over a wide angle from the material being probed, the energy analyzer should have a wide aperture and a wide acceptance angle to thereby increase the collection efficiency. A typical requirement of a commercial electron energy analyzer is that it be able to analyze 10 to 20% of the electrons emitted from the material and to distinguish electrons whose energies differ by as little as 0.1%.
Commercial energy analyzers should be rugged, small, easy to operate, and relatively inexpensive. If these commercial characteristics can be improved, materials analysis equipment can more readily find acceptance in production environments, such as in-line processing monitors in the semiconductor industry. Such characteristics are also important for remote operation, such as the search for life on Mars. For space applications, an energy analyzer needs to be lightweight, a characteristic also desired for other applications.
Several types of charged particle energy analyzers are common. Dispersive analyzers are the most common. They depend upon electric or magnetic fields to spatially deflect a well defined beam of charged particles. The amount of spatial deflection depends upon the energy (velocity) of the charged particle. A detector positioned at a particular offset from the original beam direction detects only the charged particles of an energy associated with the position. Either the detector position or the electric or magnetic field can be varied to measure an energy spectrum. Dispersive analyzers can be made designed with very high resolution. However, dispersive electron energy analyzers tend to be large and heavy, having a diameter of 1 m and a volume of 0.5 m3. In addition, they generally have a limited spatial and angular acceptance for electrons coming from a material surface. Their throughputs are generally low so they are more suited for a scientific experiment than for a industrial application.
Non-dispersive analyzers, on the other hand, rely upon high-pass and low-pass energy filters to allow only charged particles in an energy band to reach the detector. Non-dispersive electron analyzers have larger throughputs and tend to be smaller than dispersive electron analyzers. However, their energy resolution and stability have limited their use.
An early analyzer which incorporates both dispersive and non-dispersive sections is described by in “A nondispersive electron energy analyzer for ESCA,” Review of Scientific Instrumentation, vol 44, no. 7, July 1973, pp. 893–898. In it, charged particles are injected along the axis of a cylindrical chamber using a dispersive filter that eliminates very high and very low energy charged particles. In the chamber, a reflective low-pass filter is followed by a high-pass filter to select a range of energies that reach a detector. This analyzer has been used commercially in surface analysis instruments manufactured by the duPont Company and later by Shimatsu Inc. It has a higher throughput than similarly sized dispersive analyzers, but considerably less than that theoretically possible in a completely nondispersive analyzer.
A more recent nondispersive analyzer is described by Tepermeister et al. in “Modeling and construction of a novel electron energy analyzer for rapid x-ray photoelectron spectroscopy spectra acquisition,” Review of Scientific Instrumentation, vol. 63, no. 8, August 1992, pp. 3828–3834. The Tepermeister analyzer includes a high-pass filter followed by a lens which acts as a low-pass filter. It also has a high throughput, but the collection efficiency of the analyzer must be compromised considerably to achieve good energy resolution.
It is thus desired to provide a light and compact charged particle analyzer with high collection efficiency.